1. Field of the Invention
The invention relates to a method for the control of display screens, enabling an increase in the dynamic range of adjustment of the luminosity of these screens. The invention can be applied to internal memory type screens. By internal memory screens is meant screens wherein the cells that form the picture elements preserve the "written" state in which they are liable to be activated, after the end of the "written" state command signal as is the case, notably, with plasma display panels and, especially, ac plasma display panels.
2. Description of the Prior Art
The use of display screens in surroundings where the levels of luminosity are subject to very great variations may require the overall luminance of these screens to be adjusted according to the ambient luminosity in which they are used. In fact, it is recommended that the luminosity of the screen be comparable to that of the environment, without which the user will be subjected to unnecessary fatigue.
The lighting conditions around the screen may vary by a factor in the range of 1,000 (from some tens of lux in interior surroundings wit attenuated lighting to some tens of thousands of lux in external surroundings, as in bright sunlight).
This raises the problem of the dynamic range of adjustment of the overall luminance of these screens for, to date, this dynamic range is far smaller than that of the ambient luminosity, under the different possible conditions of use mentioned here above.
In the example of ac type plasma display panels, the standard method used to adjust the luminance of the screen consists in adjusting the frequency of signals, called sustaining signals, by which cells in the so-called "lit" or "written" state produce light.
The working and structure of ac type plasma display panels (which have a memory effect) are well-known per se. These panels are, for example, of the type having two electrodes in intersection to define a discharge cell, as described for example in a French patent, filed on behalf of THOMSON CSF and published under No. 2 417 848. These panels may also be of the coplanar sustaining type in which, for a single cell, addressing discharges and sustaining discharges are set up between different electrodes, as described notably in a European patent application No. EP-A-01035 382.
The principle and working of these ac type plasma display panels turns their property of memory to advantage in order to temporally separate the addressing functions (the writing or erasure of the information) of the cell from the functions used to produce the useful light.
These panels have a plurality of cells generally arranged in lines and columns. A given cell is addressed by the selection of two intersecting electrodes to which, at a given instant, appropriate voltages are applied so that the potential difference causes a writing discharge or an erasure discharge between these electrodes.
A standard addressing method uses a line-at-a-time operation. In this case, all the cells of a line are commanded simultaneously ("half-select" or semi-selective operation) to be "written on" or "erased", for example erased, and this operation is followed by a selective operation during which one or more selected cells of this same line are "written" on.
The semi-selective operation followed by the selective operation is accomplished for each line, with a time lag from one line to the next that corresponds to the duration of a line cycle.
It must be noted that the addressing by semi-selective and selective Operation is generally done by the superimposition of addressing square wave signals on basic square wave signals, as is explained for example in the French patent applications No. 88 11 247 filed on behalf of THOMSON CSF which should be considered as forming part of the present patent application, and in the French patent application No. 88 11 248 also filed on behalf of THOMSON-CSF.
These basic square wave signals are applied simultaneously to all the cells for a period of time that constitutes an addressing phase, and the addressing square wave signals are superimposed on these basic square wave signals only for the addressed lines with, from one line to another, the time lag corresponding to the duration of a line cycle CL. This means that the starting points of two consecutive addressing phases are separated by the duration of the line cycle.
Generally, in each line cycle, the addressing phase is followed by a sustaining phase during which the cells in the "written" state are activated, i.e. they produce light. Indeed, in the sustaining phase, sustaining signals are applied simultaneously to all the cells and prompt sustaining discharges that give the essential part of the emission of light perceived by the observer.
The sustaining signal is an ac signal constituted by voltage square waves that succeed one another with opposite polarities: each change in the sign of the ac signal (rising edge or trailing edge) causes a discharge in the gas and an emission of light at the cell or cells concerned. Thus, the quantity of light emitted by a cell in the "lit", i.e. "written" state is substantially proportional to the number of fronts corresponding to changes in polarity, and consequently to the frequency of the sustaining signal.
It must be noted that, in the addressing phase, the basic square waves, for the writing as well as for the erasure, have substantially the same amplitude as the sustaining signals and, consequently, they too may cause discharges comparable to the sustaining discharges, with light emission. Consequently, it may be considered that the addressing phases contain at least one sustaining cycle.
The frequency of the sustaining signal may be made adjustable and, when it is adjusted, the overall luminance of the screen is adjusted.
In practice, however, the information refreshing rates, namely the rates at which the image is renewed, as well as physical limits on the duration of the discharges, greatly restrict the possibilities of adjustment of the luminance of the screen by means of the variation in frequency of the sustaining signal.
For example, in the case of a standard plasma panel screen having 480 lines of cells, refreshed 50 times per second, i.e. with a frame period equal to 20 ms, the period of one line cycle CL corresponds to: EQU 20 ms/480=41.7.mu.s.
In practice, about 20 .mu.s are needed to carry out an addressing (during the addressing phase) that includes a half-select or semi-selective operation followed by a selective operation, and the time available within the line cycles for a phase specific to the sustaining cycles is equal to: EQU 41.7.mu.s-20.mu.s=21.7.mu.s.
FIGS. 1a and 1b, which should be looked at together, are graphs showing the distribution in time of these different phases, for only two consecutive lines Li and Li+1.
These two lines (but also all the 480 lines) simultaneously receive basic square waves (not shown) from an instant t0 onwards. These basic square waves form an addressing phase PA1. In the addressing phase, from the instant t0 to the instant t1, there is a period of erasure CE intended for command for erasure by a semi-selective operation followed, from the instant t1 onwards, by a writing period CI intended for a command for writing by a selective operation. The writing period CI ends at an instant t2 that also marks the end of the addressing phase PA1.
The addressing phase PA1 is followed by a sustaining phase PE1 that ends at an instant t3 when a second addressing phase PA2 starts. From the instant t0 onwards, the first addressing phase PA1 and the sustaining phase PE that follows defines a first line cycle CL1 that ends at the instant t3 when a second line cycle CL2 starts, and so on until a cycle CLn. All these line cycles CL1, CL2, CLn are set up in the same way.
Assuming that the addressing of the cells of the lines Li is done in a first line cycle CLl (during the first addressing phase PAl], the addressing of the line Li+1 is done during the second addressing phase PA2 of the second line cycle CL2. The addressing that follows for the line Li is then done 480 line cycles after the first cycle CLl, during the cycle CLn for example. In FIGS. 1a, 1b, the fact that the addressing has been done during a given addressing period is symbolized by hatched lines in the square waves that represent the addressing phases PA1, PA2, . . . PAn.
As mentioned further above, since the duration of an addressing phase PA1, PA2, PAn is 20 .mu.s, in the case of the example chosen, this addressing phase may be followed by a sustaining phase PE1, PE2, PEn, the duration of which is equal at the maximum to 21.7 .mu.s.
At least 5 .mu.s are needed to achieve a cycle of sustaining signals, so that a sustaining phase may include 0 to 4 sustaining cycles (at the maximum), to which there is added a sustaining cycle contained in the addressing phase PA1, PA2.
Under these conditions, the mean sustaining frequency may be adjusted between substantially 24 KHZ (that is, 1+0 /41.7 .mu.s) and 120 KHz (that is 1+4 / 41.7 .mu.s).
The dynamic range of adjustment of the luminance that may thus be obtained by adjusting the frequency of the sustaining signals corresponds to a factor 5, and it is therefore fairly low. The dynamic range may be even further reduced for screens having a greater number of lines: indeed, when the total addressing time becomes equal to the frame period (which would be obtained with 1,000 lines in the example explained here above), any possibility of adjusting the luminance by this method is eliminated.